Francis spoke to reporters on the papal plane en route from Rome to Poland

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Pope Francis arrives in procession at St. Peter's Square during the Palm Sunday Mass on March 20, 2016 in Vatican City, Vatican.

The world is at war, but it is not a war of religions, Pope Francis said Wednesday as he traveled to Poland on his first visit to Central and Eastern Europe in the shadow of the slaying of a priest in France.

The killing of an 85-year-old priest in a Normandy church on Tuesday added to security fears surrounding Francis' five-day visit for the World Youth Day celebrations, which were already high due to a string of violent attacks in France and Germany. Polish officials say they have deployed tens of thousands of security officials to cover the event.

Francis spoke to reporters on the papal plane en route from Rome to Poland. Asked about the slaying of the priest, Francis replied: "It's war, we don't have to be afraid to say this."

After greeting reporters on his papal plane he returned to the topic to clarify that when he speaks of war, he is speaking of "a war of interests, for money, resources. ... I am not speaking of a war of religions. Religions don't want war. The others want war."

Upon arrival at Krakow airport a pensive Francis was greeted by Poland's President Andrzej Duda, First Lady Agata Kornhauser-Duda and other state officials, and hundreds of faithful who had waited for hours to see him.

The main welcoming ceremony with speeches is to be held later in the day at the Wawel Castle in Krakow.

In the evening Francis is to appear in the window of the residence of Krakow bishops, where he will be staying, and chat with some among the hundreds of thousands of pilgrims from around the world gathered for the World Youth Day celebrations that will run through Sunday.

"Let's live WYD (World Youth Day) in Krakow together!" the pontiff tweeted before departing from Rome for Krakow, southern Poland, on Wednesday afternoon.

Just hours before Francis' arrival for the major Catholic event, groups of cheerful young pilgrims were seen in the streets of Krakow.

The sense of expectation was apparent in sunny Krakow on Wednesday with papal white-and-yellow flags and images of Francis and John Paul II decorating the streets. Stages were put up at many locations for concerts and other activities that are being held by and for the pilgrims in Krakow.

There was a heavy presence of police and other security forces across the city, as crowds were increasing everywhere.

"I have never seen so many people in Krakow, it's difficult to move around even though offices have closed (for the event) and many people have left the city," said Anna Gazda, 43, owner of a souvenir shop.